Low friction coatings for dynamically engaging load bearing surfaces

ABSTRACT

A gate valve has a body, the body having a cavity and a flow passage intersecting the cavity. A seat ring is mounted to the body at the intersection of the flow passage and the cavity, the seat ring having an engaging face. A gate in the cavity has an engaging face that slidingly engages the face of the seat ring while being moved between open and closed positions. A friction-resistant coating is on at least one of the faces.

RELATED APPLICATIONS

This application is a continuation-in-part of patent application U.S.Ser. No. 11/214,433, filed Aug. 29, 2005, now U.S. Pat. No. 7,255,328,which claimed the benefit of provisional patent application U.S. Ser.No. 60/605,176, filed Aug. 27, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to low friction coatings formed on loadbearing surfaces that slidingly engage each other, such as a gate andseat ring of a gate valve for a wellhead assembly.

2. Background of the Invention

Gate valves are used when a straight-line flow of fluid and minimum flowrestriction are required. When the valve is wide open, the gate is drawninto the opposite end of the valve cavity. The gate has an opening forflow through the valve the same size as the pipe in which the valve isinstalled. The valve provides an unobstructed passageway when fullyopen. It is best suited for main fluid supply lines and for pump lines,and is often used for oil and gas production where pressures may rangefrom 5000 to 30,000 psi.

Previous versions of gate valves have featured a coating on the exteriorsurface of the valve's gate and seats for reducing friction, as well asto reduce corrosion and improve wear resistance. Some previous versionshave utilized layers of hard facing, such as tungsten carbide, upon thesurface of the valve's gate and seats. Other previous versions haveutilized a vapor deposition process or a chemical vapor deposition tocoat the exterior surface of the valve's gate and seats.

Prior art gate valves rely on liquid lubrication to minimize theadhesive forces between these materials. Liquid lubricants, such ashydrocarbon and silicone based greases, decrease in both viscosity andsurface tension as their temperature is increased, thereby minimizingthe protective boundary layer they offer to the highly loaded surfaces.Additionally, only very expensive greases are stable to temperaturesabove 400 F and may lose some of their mass and lubricating properties.The loss of lubrication at high temperatures leads to significantincreases in valve torques and may lead to the galling of the matingsurfaces.

Polymer coatings have been used on sliding load bearing surfaces ingeneral, including on ball valves. Some polymer type coatings have beenused on gate valves as well, but suffer from insufficient load bearingcapacity and ductility especially at elevated temperatures. Athermoplastic polymer coating tends to creep and flow under high contactstress and elevated temperatures. A thermoset type of polymer coatingdoes not soften with temperature as does a thermoplastic, but suffersfrom poor ductility and a propensity toward greater adhesion especiallyat elevated temperatures. These properties generally result in cracks inthe coating and the removal of the coating to its mated surface.

SUMMARY OF THE INVENTION

In this invention, an apparatus for a well has first and secondcomponents, each having a metal engaging surface that engages the otherin a load bearing sliding contact. A polymer coating is formed on atleast one of the surfaces. Preferably, the polymer coating contains aquantity of stiffening particulates having average diameters less than0.5 microns, such as nanotubes.

The polymer coating is preferably a thermoplastic material. Also, in oneembodiment, the surface containing the coating has a hardened layerunder the coating. The hardened layer might be formed by nitriding,nickel aluminiding, boronizing, or carburizing. The coating ispreferably applied by spray dispersion at room temperature.

An apparatus for controlling well fluids, includes a gate valve having abody. The body has a cavity and a flow passage intersecting the cavity.A seat ring is mounted to the body at the intersection of the flowpassage and the cavity. The seat ring has an engaging face formed of asteel alloy. A gate is in the cavity and has an engaging face formed ofa steel alloy that slidingly engages the face of the seat ring whilebeing moved between open and closed positions. A friction-resistantcoating is on at least one of the faces.

In the apparatus the at least one of the engaging faces of the gate andthe seat ring can have a hardened outer layer. The friction-resistantcoating can be on the hardened outer layer. The friction-resistantcoating can include molybdenum disulfide. The friction-resistant coatingcan include tungsten disulfide. The friction-resistant coating caninclude a carbon or diamond-like material. The friction-resistantcoating can have a thickness in a range of between about 2 and about 8microns. The friction-resistant coating can have a hardness of at leastabout 900 on the Vickers scale. The friction-resistant coating can havea hardness in the range of about 900 to about 5000 on the Vickers scale.In the preferred embodiment, the friction-resistant coating has ahardness of at least about 500 on the Vickers scale, and in the range ofabout 500 to about 5000 on the Vickers scale. In the apparatus, thefriction-resistant coating can have an unlubricated coefficient offriction in a range between about 0.03 and about 0.15 and a lubricatedcoefficient of friction in a range between about 0.01 and about 0.15.

In the apparatus according, the engaging face that has thefriction-resistant coating can have a hardened layer under the coating.The hardened layer can include one of the following: a nitrided layer, anickel aluminided layer, a boronized layer, and a carburized layer.

Another aspect of the invention includes a method of forming a coatingon a metal load bearing surface. The method includes the step ofproviding a gate valve assembly having a valve body with a cavity and aflow passage intersecting the cavity. A seat ring is mounted to the bodyat the intersection of the flow passage and the cavity. The seat ringhas an engaging face formed of a steel alloy. A gate is in the cavityand has an engaging face formed of a steel alloy that slidingly engagesthe face of the seat ring while being moved between open and closedpositions. The method also includes the step of hardening at least oneof the engaging faces. The method also includes the step of applying afriction-resistant coating to the hardened engaging face.

The hardening step can include hardening the engaging face through aprocess selected from nitriding, aluminiding, nickel aluminiding,boronizing, and carburizing. The application step can include that thefriction-resistant coating is applied through a process selected fromphysical vapor deposition and chemical vapor deposition.

Another aspect of the invention includes another method of forming acoating on a metal load bearing surface. The method includes the step ofproviding a gate valve assembly having a valve body with a cavity and aflow passage intersecting the cavity. A seat ring mounted to the body atthe intersection of the flow passage and the cavity. The seat ring hasan engaging face formed of a steel alloy. A gate is in the cavity andhas an engaging face formed of a steel alloy that slidingly engages theface of the seat ring while being moved between open and closedpositions. The method also includes the step of hardening at least oneof the engaging faces. The method also includes the step of applying acoating selected from a group consisting of molybdenum disulfide andtungsten disulfide to the hardened engaging face until the coating has athickness in a range between about 2 microns and about 8 microns.

The hardening step can include that hardening the engaging face througha process selected from nitriding, aluminiding, nickel aluminiding,boronizing, and carburizing. The application of the coating step caninclude that the coating is applied through a process selected fromphysical vapor deposition and chemical vapor deposition.

The method can also include the step of applying a lubricant to thecoating after the coating is applied to the hardened engaging face.Prior to the hardening step, the method can include the step oftexturing the engaging surface being hardened to create a texturedsurface finish. The application of the coating step can also includethat the coating is applied to the textured and hardened engaging facewith the textured surface enhancing the application of the coating tothe hardened engaging face.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a gate valve having a polymercoating on at least one of the interfaces between the gate and seats inaccordance with the invention.

FIG. 2 is a schematic enlarged sectional view of the gate of the valveof FIG. 1, illustrating a hardened layer and a polymer coating, assprayed onto gate and prior to heating.

FIG. 3 is a schematic enlarged sectional view of the gate as shown inFIG. 2, but after heat processing the polymer coating.

FIG. 4 is a schematic enlarged sectional view of the gate as shown inFIG. 3, but showing an alternate embodiment of the polymer coating.

FIG. 5 is a schematic enlarged sectional view of an alternate embodimentof the gate of the valve of FIG. 1, illustrating a hardened layer and alow friction coating.

FIG. 6 is a schematic enlarged sectional view of an additional alternateembodiment of the gate of the valve of FIG. 1, illustrating anintermediate coating and a low friction coating.

FIG. 7 is a photograph of a textured surface used to improvetribological performance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, gate valve 11 has a body 13 and a flow passage 15that extends transversely through body 13. Valve 11 has a gate 17 with ahole 19 therethrough. Gate 17 is shown in the open position. The gatevalve 11 shown in FIG. 1 is a non-rising-stem type valve, but the valve11 may alternatively be a rising-stem type valve. Also shown in FIG. 1are ring-shaped valve seats 21, which have holes 23 that register withthe flow passage 15 of the valve. Gate valve 11 is shown as a split gatetype having two separate slabs, but it could alternatively be a singleslab type.

When gate 17 is in the open position, the hole 19 of the gate 17registers with flow passage 15 of the valve 11, thereby allowing flowthrough the valve. When the gate is closed, the hole 19 no longerregisters with the flow passage 15. The gate 17 has an engaging face 25on each side that interfaces with seats 21. While gate 17 is closed,typically pressure in the flow passages 15 creates a substantial load onone of the faces 25 against one of the seats 21. Movement of gate 17 toor from the closed position causes one of the faces 25 to slide againstone of the seats 21 while exerting contact forces, if one of flowpassages 15 is under high pressure. The gate valve 11 shown in FIG. 1 isa forward acting gate valve meaning that gate 17 moves down to close thevalve. Alternatively, the valve could be a reverse acting gate valve byrepositioning the location of the gate opening.

Gate valve slab or gate 17 is preferably made from corrosion resistantsteel alloys such as one of the following: Inconel (a nickel-chromealloy of steel); high quality low alloy steel; stainless steel;nickel-cobalt alloy steel; or another suitable metal material. Inconel625 typically has a Rockwell Hardness Number (HRN) in the C scalebetween 28 and 33. Inconel 718 typically has a Rockwell Hardness Number(HRN) in the C scale between 35 and 40. Material properties can bealtered by the heat treatment process. Seats 21 may be formed of thesame types of material.

Referring to FIG. 2, in one embodiment, each gate face 25 is subjectedto an optional hardening process to create a hardened layer 27 beforeapplying a low friction coating 29. The hardening process may includevarious surface hardening techniques or diffusion processes such asnitriding, aluminiding or nickel aluminiding, boronizing, orcarburizing.

Nitriding is a case-hardening process whereby nitrogen is introducedinto the surface of a solid metal alloy by holding the metal at asuitable temperature in contact with a nitrogenous substance such asammonia or nitrogen rich salt. Nitriding includes placing the gate 17within a chamber or vat and heating the gate 17. The liquid or gas typenitriding temperature for steels is between 495 and 565° C. (925 and1050° F.). At high temperatures, the nitrogen migrates into the metaland reacts to elements within the metal alloy to form a ceramic compoundof nitride. The nitrogen most effectively reacts with titanium,chromium, or other suitable elements. Ion nitriding or Plasma AssistedCVD nitriding may be carried out at lower temperatures.

Aluminiding and boronizing follow a similar procedure whereby aluminumand boron, respectively, are introduced to the part at elevatedtemperatures. In vapor-phase aluminiding procedures, the evaporatealuminum introduced into the chamber reacts most effectively withnickel. In boronizing procedures, the boron introduced into the chamberreacts most effectively with iron. After the nitriding, aluminiding,boronizing, or other hardening procedure is performed on faces 25 ofgate 17, the hardened layer 27 generally extends into the faces 25 ofgate 17 for a depth in the range of 0.0005 inches to 0.003 inches.Coating 29 preferably has a thickness of about 0.001 or more.

Before the low friction coating 29 is applied, the surface is preferablytextured slightly to create better adhesion for coating 29. Thetexturing procedure may occur before creating hardened layer 27 orafter. The texturing procedure may be performed in a variety of ways,and is performed in one technique by a combination of sand blasting andsanding or lapping. For example, face 25 may be bead blasted with 60grit beads, then sanded with 400 grit sandpaper. The purpose of sandingor lapping is to lower the peaks creating by the bead blasting step.Ideally, the average depths from valley to peak after sanding will beless than the thickness of the subsequent low friction coating 29 sothat the peaks would be covered by coating 29. Optionally, the sandingor lapping step could be followed by another step of bead blasting, butusing a smaller size of beads than in the first bead blasting step.

As an alternate to bead blasting and sanding or lapping, the surface ofgate face 25 could be textured by creating a porous surface. This couldbe done by direct application of a laser to the metal alloy of gate face25 to create small cavities. Additionally, micro-jets of water can beused to texture the surface as well as a variety of chemical etching ormilling techniques. Alternately, a porous nickel coating or a thermalspray coating, such as a WC/Co system, could be applied.

Preferably low friction coating 29 comprises a high temperature polymersuch as one of the following: PEEK (polyetheretherketone); PEK(Poletherketone); PFA (Perfluoroalkoxy); PTFE (polytetrafluoroethylene);FEP (fluorinated ethylene propylene); CTFE(polychlorotrifluoroethylene); PVDF (polyvinylidene fluoride); PA(Polyamide); PE (Polyethylene); TPU (Thermoplastic Elastomer); PPS(Polyphenylene Sulfide); PC (Polycarbonate); PPA (Polphthalamide); PEKK(Polyetherketoneketone); TPI (Thermoplastic Polyimide); PAI(polyamide-imid); PI (polyimide) or others. Preferably, the polymer is athermoplastic, but a thermoset plastic could also be employed. Athermoplastic is defined herein as a polymer that can be repeatedlyheated to its melting point. PEEK is therefore, for example, athermoplastic and PAI is not. The preferred polymers are capable ofwithstanding temperatures up to 450 degrees □F. without degradation.

Also, the preferred polymers have a high strength under compressiveloading. For example, some gates 17 must be capable of withstanding upto 60,000 psi of bearing stress between the seat and gate. If coating 29has a compressive strength below that amount, it will tend to creep orbecome semi-liquid under high pressure. The tendency to creep ispromoted as the operating temperature increases. If sufficient creepoccurs, the textured subsurface of coating 29 will penetrate the topcoating leading to the scratching of the mating surface, resulting in anincrease in friction, an increase in coating wear, and an increase inpotential leakage. Preferably, the coefficient of friction of coating 29remains below 0.03, without supplemental liquid lubrication, for atleast 200 cycles through temperature extremes to 450 F or higher.Preferably, the compressive strength is 25,000 psi at room temperaturemeasured under the test ASTM D695, 10% deflection.

One technique to impart stiffness and creep resistance to the polymer ofcoating 29 is to mix a quantity of stiffening particulates in thepolymer 33, such as nano-sized single or multi-wall nanotubes 31 ofcarbon or boron nitride. Other stiffening particulates includenano-sized fibers and micron-sized fibers such as carbon fibers.

The term “nano-sized” is used herein to mean fibers or particulates,whether tubular or solid, having a diameter of about 0.5 microns orless. Nano-sized particulates are so small that they may interact withthe molecules of the polymer, thereby imparting properties not possiblewith other additives. Property improvements may include increases increep resistance, compressive strength, tensile strength, wearresistance, abrasion resistance, tear resistance, explosivedecompression resistance, elongation to failure, and an increase in thecoatings glass transition temperature. Their small size allows them tobe sprayed with conventional dispersion coating systems. Moreover,because of the small size, the nano-sized particulates do notsignificantly affect the surface finish of coating 29. Single andmulti-wall carbon nanotubes have diameters much smaller than 0.5 micron,such as 0.015 micron. Other nano-fibers are available in size rangesapproximately 10 times larger in diameter than carbon nanotubes.Nanoceramic particulates are generally spherical and may have diametersof approximately 0.05 microns.

The term “micron-sized” as used herein refers to particulates, whetherfibers or granules, having diameters greater than 0.5 microns. Forexample, a carbon fiber might have a diameter of 8 microns. Coating 29in the embodiment of FIGS. 2 and 3 contains a quantity of carbonnanotubes 31 as well as some micron-sized carbon fibers 35, whilecoating 29′ in FIG. 4 does not contain micron-sized carbon fibers 35.Carbon fibers 35 have greater lengths than the lengths of nanotubes 31;for example 150 microns versus about 20 microns for carbon nanotubes 31.

It is also beneficial to add lubricating additives to the coatingmixture prior to application to reduce friction. The negativeconsequence of adding lubricants is to reduce the creep resistance ofthe coating system. This further increases the need for the creepresistance stiffening additives of the invention. Preferred lubricantsmay include particulates of polytetrafluoroethylene, molybdenumdisulfide, graphite, tungsten disulfide, boric acid, boron nitride,fluorinated ethylene propylene, and perfluoroalkoxy.

Coating 29 is preferably applied by a dispersion technique through aconventional paint spray gun. A quantity of nanotubes 31 or nano-sizedparticulates are compounded with the polymer 33. The compounded materialis reduced into granules 37 (FIG. 2) of sufficiently small size to beapplied as a coating by electrostatic dispersion or thermal sprayprocesses. Granules 37 have average diameters less than about 200microns. In one embodiment, granules 37 have diameters of about 12microns. Preferably, nanotubes 31 make up at least six percent by volumeof each granule 37 to provide the desired stiffness to coating 29. Onepreferred range is from six to thirty percent by volume.

A surfactant and water are mixed with granules 37 to form a dispersion.Additives for lubrication enhancement may be added to the dispersion.Micron-sized fibers 35, such as carbon fibers, may optionally be addedto the dispersion. If so, preferably the quantity of micron-sized fibers35 by volume to nano-sized fibers 31, is about one to ten. Thedispersion mixture is sprayed onto face 25 at room temperature. Thengate 17 is placed in a furnace and heated to a temperature of about 725degrees F. The temperature is sufficient to melt polymer 31 but is belowthe first transformation temperature of the steel alloy gate 17, thusdoes not affect the hardness, whether or not a hardened layer 27 isused. Once cooled, coating 29 becomes solid, durable, and bonded to gateface 25. The longer micron-sized fibers 35, if used, act as reinforcingstrands that bind the thermoplastic granules 37, themselves filled withnano-sized fibers 31, together.

Another method of applying the coating to a part is by the use a thermalspray process. In this process the thermoplastic granules 37, filled ornot, are mixed with other solid particulates such as lubricants andlarger fibers, such as carbon fibers 35. This powder mixture is thensprayed through a gun that melts the mixture before or as it is sprayedonto the part. The part therefore does not need to be thermallyprocessed after the coating is applied.

Yet another method is to charge a dry powder mixture and apply thepowder coating to the part electrostatically. The part is subsequentlyheated to melt and bind the particulates. This process is normally usedfor thick polymer coatings

Multiple coatings may be applied to the part to impart uniqueproperties. For example a first layer with micron-sized fibers, as wellas other nano-sized particulates, may be applied to increase creepresistance and compression strength. A top coat without the fibers andparticulates may be applied to obtain low frictional properties.

While the use of a thermoplastic is discussed in some detail, many ofthe methods described herein are applicable for use with thermosetmaterials. In particular, polyamide-imid (PAI) is a polymer that can beprocessed in a solution of water or solvent. Additives can be added toachieve a wide range of properties. Nanotubes or nanofibers may be addedto the solution to improve coating properties. If dried at a lowtemperature, the PAI binder system provides for a good low temperaturecoating. When heated to about 500 F, the PAI reacts to form a polyimidematerial thereby greatly improving the thermal properties of the polymerin the coating.

Coating 29 may also be applied to the faces of seats 21 in the samemanner as described in connection with gate face 25. Coating 29 could beomitted from gate face 25, or both seat 21 and gate face 25 could have acoating 29. No hydrocarbon-based liquid lubricant or grease is requiredin conjunction with gate face 25 and seat 21. The addition of a liquidlubricant, however, can reduce the start up friction of the valvesystem.

When moving the gate 17 across the seat face 21, low friction coating 29provides for a reduced coefficient of friction, reduced wear, andgalling prevention. The approximate unlubricated or dry coefficient offriction is in the range of approximately 0.01 to 0.03 even afternumerous cycles of use. The low coefficient of friction reduces torquerequirements to cycle the gate. Wear rates are substantially reducedduring gate valve 17 operations by virtue of the coating.

Reducing the work energy and torque required to operate the gate valveeffectively extends low-cost non-rising stem designs to larger sizes andhigher pressure ratings without the use of complex gear reducers orexpensive rolling element devices. The invention enables gate valves tobetter withstand contact stresses, and provides for improved wearresistance. The invention also increases the valve's operatingtemperature. Eliminating liquid lubricants enables the gate valve toqualify for higher temperature ratings, such as 450 degrees F. Suchadvantages will provide a significant cost and performance advantageover previous versions in the art.

In addition to applying coatings as described to components of a gatevalve, there are other applications, particularly in connection with oiland gas well surface equipment. For example, threads of high loadfasteners may contain such a coating. Fasteners of this category includebolts used to fasten sections of offshore drilling riser together.Coatings of the type described could also be used on ball valves andtensioners for tensioning offshore riser strings.

In an alternative embodiment shown in FIGS. 5 and 6, an exterior portionof face 25′ is subjected to an optional a hardening process to create ahardened layer 27′ before applying a low friction or friction-resistantcoating 29′. The hardening process may include various surface hardeningtechniques or diffusion processes such as nitriding, aluminiding, nickelaluminiding, boronizing, or carburinzing, as discussed above herein, orthrough thermal spraying, cladding, or electroplating. Examples ofmaterials that can be applied through a thermal spray process includetungsten carbide in a cobalt matrix, or silicone carbide in a cobaltmatrix.

Before the low friction coating 29′ is applied, face 25′ is preferablytextured slightly to create better adhesion for coating 29′. One methodof texturing face 25′ is that a laser may be utilized on the face 25′for defining very small cavities extending approximately 0.001 inchesinto the face 25′ of gate valve 17. The laser application can be usedwith both lubricated surfaces or non-lubricated surfaces, and can beperformed either before or after the hardening process.

After the exterior face 25′ is hardened as in FIG. 5, low frictioncoating 29′ is applied thereupon. The low friction coating 29′ isapplied by way of physical vapor deposition (PVD), chemical vapordeposition (CVD), or alternatively by another binderless spray process.Low friction coating 29′ is primarily made from carbon or diamond-likematerial, molybdenum disulfide, tungsten disulfide, or another suitablematerial. Low friction coating 29′ is preferably thin, having anapproximate thickness in the range of 2 microns to 8 microns.

Low friction coating 29′ forms a hard layer, sometimes having a hardnessgreater than the hardened layer 27′. The hardness of low frictioncoating 29′ may be in the vicinity of 900 to 5000 or more on the Vickersscale.

There are various known processes for applying low friction coating 29′.One technique, described in U.S. Pat. Nos. 4,987,007 and 5,098,737,creates amorphous diamond coatings. In this process, ions are extractedfrom a laser ablation plume in a vacuum environment at room temperature.The ions are accelerated through a nozzle for deposit on a substrate.Other processes utilize various Chemical Vapor Deposition processes suchas plasma assisted CVD.

In operation, when moving the gate 17 across the seat face 21, thehardened surface and the low friction coating 29′ provide for a reducedcoefficient of friction, reduced wear, and galling prevention. Theapproximate unlubricated or dry coefficient of friction is in the rangeof approximately 0.03 to 0.15, and the approximate lubricatedcoefficient of friction is in the range of 0.01 to 0.1. Thesecoefficients of friction and wear rates are substantially reduced duringgate valve 17 operations by virtue of the hardened face 25′ coupled withthe low friction coating 29′. Textured surfaces of the mating partsfurther reduce friction and wear by minimizing solid surface contact byenhancing the ability of the lubricant film to support the load. Arepresentative view of a textured surface is shown in FIG. 7.

An alternative procedure to the diffusion processes described above isapplying an intermediate coating 47, without performing theaforementioned heat treatment hardening process. The intermediatecoating 47 would be applied to the exterior face 25′ of the gate 17,after which the low friction coating 29′ would be applied on top of theintermediate coating 47. In this manner, the intermediate coating 47 maysubstitute for the hardened layer 27′ of the gate 17, or alternativelythe intermediate coating 47 may be utilized in conjunction with hardenedlayer 27′. The intermediate coating 47 is typically made from suchhardened materials such as titanium nitride (TiN), Chromium Nitride(CrN), Titanium Aluminide (TiAl), or other sufficiently hardenedmaterial. The intermediate coating 47 may also be an electroless orelectro-deposited type coating. The intermediate coating 47 is generallyin the approximate range of 2 microns to 8 microns in thickness, but maybe as much as 50 microns.

The invention has several important advantages. The low frictioncoatings reduce the valve work and torque by minimizing the frictionalsliding forces at the gate to seat interface and at the stem to drivebushing interface. Reducing the work energy and torque required tooperate the gate valve effectively extends low-cost non-rising stemdesigns to larger sizes and higher pressure ratings without the use ofcomplex gear reducers or expensive rolling element devices. Theinvention enables gate valves to better withstand contact stresses, andprovides for improved wear resistance. The invention may also increasethe valve operating temperature. Such advantages will provide asignificant cost and performance advantage over previous versions in theart.

Although some embodiments of the present invention have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made hereupon without departing from theprinciple and scope of the invention. Accordingly, the scope of thepresent invention should be determined by the following claims and theirappropriate legal equivalents.

That claimed is:
 1. A method of manufacturing a valve, comprising:thermally spraying tungsten carbide in a cobalt matrix to a selectedload bearing surface of a component of the valve to produce a hardenedlayer on the load bearing surface; applying a low friction coating ofdiamond-like carbon layer to the hardened layer; assembling thecomponent in the valve with the diamond-like carbon layer in slidingengagement with an engaging surface of the valve; and wherein the loadbearing surface comprises an engaging face of a seat ring of the valveand the engaging surface comprises a face of a gate of the valve that ismoved linearly across the engaging face on the seat ring.
 2. The methodaccording to claim 1, further comprising applying a lubricant to thediamond-like carbon layer.
 3. The method according to claim 1, furthercomprising the step of texturing the load bearing surface prior toapplying a thermal spray to create a textured surface finish.
 4. Themethod according to claim 1, wherein the face of the gate comprises asteel alloy that is free of a diamond-like carbon layer.
 5. A method ofmanufacturing a valve, comprising: thermally spraying tungsten carbidein a cobalt matrix to a surface of a valve component to deposit ahardened layer; applying a diamond-like carbon layer to the hardenedlayer on the surface of the valve component using a vapor depositionprocess; assembling the valve component in the valve with thediamond-like carbon layer in sliding engagement with a steel alloysurface of the valve; and wherein the valve component comprises a seatring and the steel alloy surface comprises an engaging face of a gatethat is moved linearly across the diamond-like carbon layer on the seatring.
 6. The method as recited in claim 5, further comprising applying alubricant to diamond-like carbon layer.
 7. A method of manufacturing agate valve, comprising: thermally spraying tungsten carbide in a cobaltmatrix to an engaging face of a seat ring of the gate valve to produce ahardened layer; applying a low friction coating comprising adiamond-like carbon layer to the hardened layer on the engaging face ofthe seat ring of the gate valve using a vapor deposition process;installing the seat ring in the gate valve; installing a gate within thegate valve, the gate having a steel alloy surface; applying a grease toat least one of the steel alloy surface and the diamond-like carbonlayer; and moving the gate linearly with the steel alloy surface slidingacross the diamond-like carbon layer on the engaging face of the seatring.